The present invention relates to variable speed induction motors and the use of such in barrier movement operators.
Barrier movement operators, such as gate and garage door operators comprise an electric motor connected to move a barrier between at least open and closed limits. In controlling the movement of the barrier, the motor and the barrier must be started in motion from rest and stopped from motion into the rest state. Sometimes, as is the case when an obstruction is in the path of the barrier, the barrier must be stopped then started again in a reverse direction. When the electric motor moving a barrier stops and/or starts abruptly the inertia of the at rest or moving barrier creates large forces. Such forces potentially reduce the lifetime of the barrier movement operator and create audible and visual appearances that the barrier movement operator is straining. Also, it may be desirable to move the barrier at different speeds during travel along different parts of the path of travel. For example, it may be desired to open the barrier at a faster speed than when it is closing.
Some DC motor powered systems, such as that described in PCT/US02/24385, build up and diminish the power (torque) applied by the motor when the motion of the barrier is changed. Such building-up and diminishing may be done by timed increases and decreases of a DC voltage level or by pulse width modulating the DC power. DC motors require a powerful and costly source of DC voltage, relatively complex control circuitry and the more expensive DC motor itself. A need exists for methods and apparatus to control the power to an A.C. induction motor of a barrier movement operator in response to barrier control input signals.